1. Field of the Invention
This invention relates to a bipolar IC employed in a variety of linear circuits. More particularly, it relates to a band gap reference circuit capable of outputting optional voltages of good temperature characteristics by a simplified structure.
2. Description of the Related Art
In general, a bipolar IC is used widely for processing electrical signals of equipment for household and industrial application. As a constant voltage source of the bipolar IC, a band gap reference circuit of good temperature characteristics is used extensively. FIG. 1 shows an example of this band gap reference circuit.
A transistor 101 has its emitter grounded, while having its base connected to its collector, and to the base of a transistor 102. The transistor 102 is a parallel connection of n NPNs and has its emitter grounded via a resistor 109 while having its collector connected to a resistor 111 and to the base of a transistor 103. The transistor 103 has its emitter grounded, while having its collector connected to the collector of the transistor 106 and to the collector of a transistor 107.
A transistor 104 has its emitter connected to the resistor 111 and to a positive input of an operational amplifier 117, while having its collector connected to the base of a transistor 105 and to the base of the transistor 106. The transistor 105 is a parallel connection of n NPNs and has its emitter connected via a resistor 112 to the positive terminal of a power source 118. The transistor 106 has its emitter connected to an emitter of the transistor 107 and a resistor 113. The base of the transistor 107 is connected to the base and the collector of the transistor 108 and grounded via resistor 114. The transistor 108 has its emitter connected to the positive terminal of the power source 118.
The negative input of the operational amplifier 117 is grounded via resistor 115, while being connected to its own output via resistor 116.
The operating principle of this circuit is hereinafter explained. The base current of the transistors is disregarded.
It is assumed that the current flowing through the transistor 101 is I1, with the current flowing through its base-emitter path being Vbe1. It is also assumed that the current flowing through the transistor 102 is I2, with the current flowing through its base-emitter path being Vbe2. If the sum current of these currents I1 and I2 is equal to 2I, the current flowing through the transistor 103 is I, by the current mirror circuit constituted by the transistors 105 and 106 and by the resistors 112 and 13. It is also assumed that the voltage across the base and the emitter of the transistor 103 is Vbe3, the resistance value of the resistor 109 is Re, the resistance value of each of the resistors 110 and 111 is R and the emitter voltage of the transistor 104 is Vo.
The voltage Vo then is represented by the following equation (1-1), with the current I being represented by the following equation (1-2):
Vo=Vbe1+R.multidot.I1=Vbe3+R.multidot.12 (1-1) EQU 2I=I1+I2 (1-2)
By the Schokley's diode equation, Vbe1 and Vbe3 are represented by the following equations (1-3) and (1-4): EQU Vbe1=Vt.multidot.1n(I1/Is) (1-3) EQU Vbe3=Vt.multidot.1n(I/Is) (1-4)
where Vt is a thermal voltage and Is is a proportionality constant.
Substituting the equations (1-2), (1-3) and (14) into the equation (1-1) and recomputing, the following equation (1-5): EQU I=I1=I2 (1-5)
is obtained, from which it is seen that equal currents flow trough the transistors 101, 102 and 103.
From this equation, the voltages Vbe1 and Vbe2 are represented by the following equation (1-6): EQU Vbe1=Vbe2+Re.multidot.I (1-6)
Also, from the Schokley's diode equation, Vbe2 is represented by the following equation (1-7): EQU Vbe2=Vt.multidot.1n{I/(n.multidot.Is)} (1-7)
Substituting the equations (1-3), (1-5) and (1-7) into the equation (1-6) and recomputing, the following equation (1-8) representing the relationship between the current I flowing through each of the transistors 101 to 103 and other constants: EQU I=(1n(n)/Re).multidot.Vt (1-8)
Substituting the equations (1-3), (1-5) and (1-8) into the equation (1-1), and computing, the following equation (1-9) representing the voltage Vo: EQU Vo=Vbe1+(R/Re).multidot.1n(n).multidot.Vt (1-9)
is obtained.
The condition under which this voltage Vo is not temperature-dependent is that the voltage Vo differentiated with respect to temperature is equal to 0. That is, it suffices if the following equation (1-10) EQU dVo/dT=(dVbe1/dT)+(R/Re).multidot.1n(n).multidot.k/q=0 (1-10)
where k is the Boltzmann's constant and q is an electron charge, holds.
It is well known that the voltage Vbe across the base and the emitter of a silicon transistor is decreased by 1.7 mV with rise in temperature by 1.degree. C. Therefore, the voltage Vo is not temperature-dependent if the respective constants are determined so that the following equation (1-11): EQU (R/Re).multidot.1n(n)=-(q/k).multidot.(dVb1/dT)=19.7 (1-11)
It is also well-known that the voltage Vbe across the base and the emitter of the silicon transistor is approximately 0.7 V in the vicinity of room temperature. Substituting this value and the value of the equation (1-11) into the above equation (19) and computing, the voltage Vo with good temperature characteristics, obtained by the band gap reference circuit, is 1.21 V.
Stated differently, the voltage Vo produced when the negative temperature characteristics of the voltage Vbe is cancelled with positive temperature characteristics of the thermal voltage Vt is 1.21 V.
The operation of other constituent portions of the band gap reference circuit is now explained briefly.
The transistor 104 operates as a part of a negative feedback circuit for stabilizing the voltage Vo. That is, if the voltage Vo is about to be increased, the base voltage of the transistor 103 is increased, with the base voltage of the transistor 104 then being about to be decreased. The result is that the voltage Vo is a stable voltage.
The transistors 107, 108 and the resistor 114 represent a startup circuit for power on of the above-mentioned band gap reference circuit. During the normal operation, the transistor 107 is turned off
For changing the above-mentioned voltage Vo to an optional magnitude, voltage conversion through a DC amplifier is required.
Such a DC amplifier may be constituted by an operational amplifier 117, a resistor 115 and a resistor 116. If the resistance value of the resistor 115 is Ri and that of the resistor 116 is Ro, the DC amplification ratio is Ro/Ri. Therefore, an optional constant voltage Vo' is given by the following equation (1-12): EQU Vo'=(Ro/Ri).multidot.Vo (1-12)
However, since the DC amplifier needs to be constituted within the bipolar IC, the number of circuit elements is increased such that the voltage Vo is worsened in precision due to variations in the resistance ratio Ro/Ri.
That is, the constant voltage source employing the conventional band gap reference circuit suffers a problem that the number of elements is increased or precision is worsened by the resistance ratio such that a desired voltage cannot be obtained accurately.